Wiring 24v panels into 12v battery bank

Welcome to the forum Heather.

First, exactly what solar panels do you have? Really need to know the Vmp and Imp ratings of the panels (voltage and current maximum power at standard test conditions). There are "different" 24 volt panel Vmp voltages (typically ~30 Volts and ~36 Volts)--And how you wire them and what charge controllers you can use depends on the answer.

Also, what is the AH rating of your Rolls 6 volt batteries?

Second, you do have several options to use your "24 volt" solar panels. The "cheap way" is to connect them in parallel to charge your 12 volt battery bank through a PWM (rated for your needs) to control the solar charging current. More or less, doing it this way will only get you ~1/2 rated Watts from your solar panels, but you can use much less expensive PWM controllers for this job.

If you will be eventually getting MPPT type charge controllers... The answer is with the correct solar panels and the correct MPPT Controller, they can give you about 95% of the panels' rated output to charge your battery bank. However, MPPT controllers (especially the larger/more feature rich ones) are not cheap at all. Plus, charging a 12 volt battery bank, the controllers typically have an output limit of ~60-80 amps for the larger controllers--This means you would need 2x as many MPPT controller so charge a 12 volt battery bank vs a 24 volt battery bank.

Power = Voltage * Current --- So if you have 2x the voltage with a fixed current, you are passing 2x the wattage through the charge controllers (true for both MPPT and PWM controllers).

One of things I like to start with is reviewing your loads/power needs (and roughly where the system will be installed to figure out how much seasonal sun and your temperature ranges).

Then design the system to support those needs.

In any case, your batteries are the heart of your system... And are you measuring/logging the Specific Gravity of each cell so you understand how well they are being charged and how deeply they are typically discharged? Improper operation (and improper watering) of your batteries can take years off of their life.

Even if you do not reconfigure to 24 or 48 volt battery bank at this time, keeping your battery bank happy is critical to not killing the batteries before their time.

-Bill

I am sorry, I cannot read the label in the photo (the present forum software is a pain when trying to post images. Anyway, is it this?

System Rating:280 Watts
Watts (PTC): 249.1 Watts
Max Power Voltage (Vmpp): 35.6 Volts
Max Power Current (Impp): 7.86 Amps
Open Circuit Voltage (Voc): 44.2 Volts
Short Circuit Current (Isc):8.42 Amps
Max System Voltage: 600 Volts
Series Fuse Rating: 15Amps
Module Efficiency: 14.59%

If so, then you are in good to be able to use 1 panel for 24 volt battery bank or 2 panels in series for a 24 volt PWM charge controller...

In general, for larger solar arrays like this, using a MPPT controller may be a better choice, if the costs do not hurt you too badly.

So, your present option is one panel in series, bunch in parallel connected either directly to the battery bank, and use the MW&S diversion/dump controller to regulate battery charge.

But, if you really want to do a better job of keeping the battery charged, then using a series type solar charge controller is really a better choice (PWM or MPPT).

OK you have two x 4 * volt batteries @ 600 AH each... That would give you a 1,200 AH @ 24 volt battery bank--That is really a huge battery bank... More or less, when a battery bank exceeds around 800 AH, it is a good idea to step up to the next voltage. In your case, a 600 AH @ 48 volt battery bank.

For example, we recommend around 5% to 13% rate of charge, for your battery bank:

• 1,200 AH * 29 volts charging * 1/0.77 panel+charge controller derating * 0.05 rate of charge = 2,260 Watt array minimum
• 1,200 AH * 29 volts charging * 1/0.77 panel+charge controller derating * 0.10 rate of charge = 4,519 Watt array nominal
• 1,200 AH * 29 volts charging * 1/0.77 panel+charge controller derating * 0.13 rate of charge = 5,875 Watt array "cost effective" maximum

So a 2,800 Watt solar array would be "OK" assuming your loads are not too large, and the wind turbine gives you some good power too.

10 panels * 7.86 Amps Imp = 78.6 Amps... You would need to get a PWM or MPPT controller rated for this power level (i.e., something like 2x60 Amp MPPT controller or 1x 60 amp MPPT minimum.

Note that nobody gets "12 hours a day of sun" like we use the numbers here... Lets pick Albuquerque New Mexico as similar to your weather/location... Using PV Watts, fixed array, tilted to 35 degrees from horizontal:

Month
Solar Radiation
(kWh/m²/day)


1
5.33


2
6.06


3
6.44


4
7.16


5
7.40


6
7.10


7
7.13


8
7.02


9
6.71


10
6.55


11
5.73


12
5.14


Year
6.48


So, you an see that you get around 5.x to 7.x hours of "noon time equivalent" sun per day--That is a lot, but not 12 hours per day.

A 2,800 watt array would generate roughly:

• 2,800 * 0.52 off grid system efficiency * 5.14 hours of sun average (December) = 7,484 WH = 7.5 kWH per day (December average)

And a 1,200 AH @ 24 volt battery bank with 2 days of storage and 50% maximum discharge will supply:

• 1,200 AH * 24 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max battery discharge = 6,120 Watt*Hours = 6.1 kWH per day

Because you have lots of sun, your system is relatively well "balanced"... My suggest would be to go with a 48 volt battery bank (reconfigure your string to 8*6 volt batteries in series)--But it does appear that it would work OK as is... I would see how it runs for you.

-Bill